The multiple sclerosis drug Tysabri (natalizumab) increases the risk of a fatal brain virus ten-fold. Progressive multifocal leukoencephalopathy (PML) is a rare disease that attacks the brain’s white matter—but it becomes more common when the immune system is compromised, as can happen when people take immunosuppressive drugs, say researchers.

People taking the drug are far more likely to test positive for the John Cunningham virus (JCV), which causes PML, researchers at the University of Muenster in Germany have found.

They tested 525 MS patients in Germany and 711 more in France who were all taking natalizumab; by the end of the 15-month trial, 43 of 339 people who had initially tested negative in the German group, and 41 of the 243 in the French group, were JCV-positive.

Only one percent of the general population and even MS sufferers not taking natalizumab are JCV-positive.